Infestation of the human body by lice is an increasingly prevalent social and health problem in many countries, including the United States. Lice are very small insects (about 2-3 mm in length). They deposit eggs either on a hair or fabric fiber and attach them firmly with a cementlike excretion. The eggs generally hatch in about six to ten days, depending on temperature. The empty shells remaining after the nymphs emerge from the eggs look like white grains of sand. These shells are called nits.
The Anoplura, or sucking lice, are parasites found on nearly all groups of mammals. Of the 15 recognized families of Anoplura, two families, Pediculidae and Pthiridae, have species found on humans. Pediculus humanus is the only species in the family Pediculidae that infests humans. It includes the head louse, Pediculus humanus capitis; and the body or clothing louse, Pediculus humanus humanus, sometimes called Pediculus corporis. The crab louse, Pthirus pubis, is a distinct species and is the only member of the family Pthiridae that infests humans. As used herein, the term "human lice or louse" includes a member of Pediculus humanus or Pthirus pubis.
Human lice are spread by crowding and common usage of clothing and combs. Initially, infestations result at most in irritation, but the irritation can lead to infection of the irritated area. There are at least three major diseases that are primarily transmitted by lice: epidemic typhus, trench fever and relapsing fever.
Although the human lice varieties are related, each of them has specific characteristics with regard to habitat and feeding. For example, head lice are small hard-shelled ectoparasites which cling to hair shafts while feeding, mating and laying eggs. The louse must remain on the head or it will die within a short period of time. Head lice proliferate at an incredible rate. A louse is ready to mate and reproduce within 10 hours after hatching. Under ideal conditions, a female louse may produce up to 300 eggs in its lifetime. Ideal conditions include an adequate food supply, environmental temperatures from about 28.degree. C. to about 32.degree. C., and relative humidity of about 70% to about 90%.
Poor hygienic and grooming habits are also known to contribute significantly to the spread of lice. Thus, lice infestations are most serious in geographical areas where the inhabitants have both substandard hygienic facilities and practices. Lice can be a problem, however, even when conditions are relatively sanitary.
The louse's hard chitinous exoskeleton serves as protection from external elements. Lice eggs (or ova) are similarly protected by a chitinous sheath surrounding the eggs and attached to the hair shaft. Although lice may be affected by the use of an insecticide, the eggs often remain resistant to attack. Thus, optimum treatment of a lice infestation includes both a pediculicide, which kills the adult lice, and an ovicide, which interrupts the development of the eggs.
Biologically active agents have been used for some time in attempts to control lice. For example, lindane (gammabenzene hexachloride), organophosphates (malathion), natural pyrethrins, and synthetic compounds known as pyrethroids (such as permethrin) have been used as pediculicides in lice treatment formulations. These agents however, have drawbacks. For example, lindane has a poor safety profile, and lice have developed resistance to it. Natural pyrethrin requires frequent follow-up treatments because it provides only short term residual action. Synthetic pyrethroids, although more effective against lice than natural pediculicides, are often more toxic to the subject being treated.
Spinosyns (also known as A83453 factors) are agricultural insecticides that have shown activity against 1) southern armyworm and other insects in the order Lepidoptera, 2) cotton aphid and other members of the order Homoptera, and 3) stable flies, blow flies and mosquitos, which are members of the insect order Diptera. (See U.S. Pat. No. 5,362,634, infra). Spinosyn A has an excellent human and animal safety and toxicological profile.
This invention provides formulations for controlling infestations of lice in a human comprising a spinosyn, or a physiologically acceptable derivative or salt thereof, and a physiologically acceptable carrier. It also provides methods of using these formulations to control human lice species. These formulations and methods control lice in a safer, more effective manner than previously known anti-lice formulations and methods.